I do not understand why this requirement was added to the service file @TrystanLea @glyn.hudson.
To fix (but you will need to undo it for future updates to work unfortunately)
sudo nano /opt/openenergymonitor/emonhub/service/emonhub.service
and replace (or comment out with #) this line
After=var-log.mount home-pi-data.mount network.target
with this line
After=var-log.mount network.target